Frequency bands and modes associated with various protocols are specified per industry standards for cell phone and mobile device applications, WiFi applications, WiMax applications and other wireless communication applications, and the number of specified bands and modes is increasing as the demand pushes. Examples of the frequency bands and modes for cell phone and mobile device applications are: the cellular band (824-960 MHz) which includes two bands, CDMA (824-894 MHz) and GSM (880-960 MHz) bands; and the PCS/DCS band (1710-2170 MHz) which includes three bands, DCS (1710-1880 MHz), PCS (1850-1990 MHz) and AWS/WCDMA (2110-2170 MHz) bands. Examples for uplink include the frequency ranges of DCS (1710-1785 MHz) and PCS (1850-1910 MHz). Examples for downlink include the frequency ranges of DOCS (1805-1880 MHz) and PCS (1930-1990 MHz). Examples of frequency bands for WiFi applications include two bands: one ranging from 2.4 to 2.48 GHz, and the other ranging from 5.15 GHz to 5.835 GHz. The frequency bands for WiMax applications involve three bands: 2.3-2.4 GHZ, 2.5-2.7 GHZ, and 3.5-3.8 GHz. Use of frequency bands and modes is regulated worldwide and varies from country to country. For example, for uplink, Japan uses CDMA (915-925 MHz) and South Korea uses CDMA (1750-1780 MHz). In this document, “modes” refer to WiFi, WiMax, LTE, WCDMA, CDMA, CDMA2000, GSM, and so on; and “bands” or “frequency bands” refer to frequency ranges (700-900 MHz), (1.7-2 GHz), (2.4-2.6 GHz), (4.8-5 GHz), and so on.
Power amplifiers (PAs) are designed to amplify power of radio frequency (RF) signals and are widely used in various RF circuits and devices. In modern communication systems, it is generally preferred that PAs provide high linearity and high efficiency in order to achieve a certain performance level. High efficiency is important for power loss reduction to prolong the battery lifetime of handsets, for example. High linearity is important to maintain the integrity of the signal with minimal distortion. Specifications on PA performances are defined for individual modes and bands per industry standards. These specifications involve properties associated with output signals, such as output power, power added efficiency (PAE), error vector magnitude (EVM), adjacent channel leakage ratio (ACLR) and other performance parameters. PAE is defined as the ratio of the difference between output power and input power to the DC power consumed. The curve showing output power versus input power indicates linearity. Linearity may also be evaluated by EVM, which is a measure of how far the points are from the ideal lattice points, expressed as a percentage. Generally, an EVM diagram illustrates that the fixed lattice points correspond to non-distortion of the signal forms and the distortions are quantized by the deviations from the lattice points. Thus, as linearity improves, the EVM value decreases. The EVM value of 0% corresponds to non-distortion, that is, the output signal from the PA has a perfect copy of the input signal, thereby giving rise to ideal linearity. The linearity specification in terms of EVM is 3% for LTE and WiFi, for example. ACLR is another performance measure for linearity and is specified for CDMA, WCDMA, LTE and WiMAX. It is defined as the ratio of the integrated signal power in the adjacent channel to the integrated signal power in the main channel. ACLR is also referred to as adjacent channel power ratio (ACPR). Transistors are used for the power amplification purposes and may be integrated on a chip. These transistor may be a Metal Semiconductor Field Effect Transistor (MESFET), a Pseudomorphic High Electron Mobility Transistor (pHEMT), a Heterojunction Bipolar Transistor (HBT) or of other suitable technologies.